1. Field of the Invention
This invention relates to wireless communication systems, and more particularly to wireless point to multi-point voice, data and video ("broadband") communication systems.
2. Description of Related Art
A wireless communication system facilitates two-way communication between a plurality of subscriber radio stations or subscriber units (either fixed or portable) and a fixed network infrastructure. Exemplary systems include mobile cellular telephone systems, personal communication systems (PCS), and cordless telephones. The objective of these wireless communication systems is to provide communication channels on demand between the subscriber units and the base station in order to connect the subscriber unit user with the fixed network infrastructure (usually a wired-line system). In the wireless systems using multiple access schemes, frames of time are the basic transmission unit. Each frame is divided into a plurality of slots of time. Some time slots are used for control purposes and some time slots are used for information transfer. Information is typically transmitted during time slots in the frame where the time slots are assigned to a specific subscriber unit. Subscriber units typically communicate with the base station using a "duplexing" scheme which allows for the exchange of information in both directions of connection.
Transmissions from the base station to the subscriber unit are commonly referred to as "downlink" transmissions. Transmissions from the subscriber unit to the base station are commonly referred to as "uplink" transmissions. Depending upon the design criteria of a given system, the prior art wireless communication systems have typically used either time division duplexing (TDD) or frequency division duplexing (FDD) methods to facilitate the exchange of information between the base station and the subscriber units. Both the TDD and FDD duplexing schemes are well known in the art.
In FDD systems, duplexing of transmissions between a base station and its subscriber units is performed in the frequency domain. Different sets of frequencies are allocated for uplink and downlink transmissions. For example, two well-known FDD systems are the pan-European GSM system (also known as Global System for Mobile Communication) and the North American IS-54 and IS-136 wireless communication systems. Both of these systems use a TDMA (time-division multiple access) with an FDD duplexing approach. See, e.g., D. J. Goodman, "Second Generation Wireless Information Networks," IEEE Trans. Veh. Tech., VT-40, No. 2, pp. 366-374, May 1991. The IS-54 air interface uses TDMA/FDD technology with three channels per 30-kHz AMPS carrier. The GSM air interface is characterized by an eight-order TDMA scheme with FDD. The available frequency band in Europe is 2*25 MHz, with radio channel spacing of 200 kHz. In both wireless systems, a base station transmits information to a plurality of subscriber units during a given first set of time slots and using a pre-determined set of downlink frequencies. Subscriber units transmit information to the base station using a pre-determined set of uplink frequencies. The uplink and downlink frequencies are offset or spaced in the frequency domain by a known spacing value. The duplex spacing in both the GSM and IS-54 systems is 45 MHz (i.e., the downlink frequency of a given subscriber unit is separated by 45 MHz from the uplink frequency of that subscriber unit).
Disadvantageously, FDD systems require frequency separation between the uplink and downlink frequency bands. The bandwidth allocation schemes needed to provide a given service are made more complex and therefore more costly than those used by TDD systems. FDD systems also disadvantageously require that a duplexor be provided with the subscriber unit antenna in order to separate the transmission and reception signals from each other at the antenna. Consequently, the complexity and costs associated with the subscriber unit are increased. Although FDD systems are effective in reducing interference between the uplink and downlink transmissions, FDD systems have limited flexibility and limited available frequency spectrum which is especially disadvantageous in broadband wireless communication systems. FDD systems allocate an equal or symmetrical bandwidth for both the uplink and downlink transmissions. However, many broadband services have asymmetrical bandwidth requirements (i.e., the percentage of downlink transmissions far outnumber the percentage of uplink transmissions, or vice versa). An FDD approach therefore results in under-utilized spectrum when used to duplex transmissions in a broadband communication system. The FDD approach also disadvantageously requires that adequate frequency spectrum be available for both uplink and downlink transmissions when converting to another frequency band for a related application.
In TDD systems, duplexing of transmissions between a base station and its subscriber units is performed in the time domain. A selected subscriber unit typically communicates with a selected base station using a specific pre-defined radio frequency. The channel is time-divided into repetitive time periods or time "slots" which are employed for uplink and downlink transmissions. In contrast to FDD systems, frequency allocation or frequency reuse patterns are simplified because there is no requirement of frequency separation between the uplink and downlink transmissions. Both the uplink and downlink transmissions occur during different pre-determined time slots using the identical radio frequency. However, the subscriber units in the TDD systems disadvantageously must accommodate an increased instantaneous bit rate which is required due to the time sharing of the channel. The modems in the subscriber units of TDD systems are typically active only one-half of the time. As a consequence, in order to achieve the same average bit-rates, the typical TDD modem is more complex than it would be in a system which would allow the modems to always remain active. Therefore, the TDD modems are more complex and therefore more costly than necessary to achieve a given average bit rate.
One well-known application for the TDD approach is found in digital cordless telephone (DCT) systems. Transmission standards or specifications have been developed in both Japan and Europe for use in designing DCT systems. Each of the transmission standards use a TDD technique for two-way communication. The Japanese DCT transmission standard specifies the use of a plurality of individual carrier signals having a frequency separation of 300 kHz within an overall system bandwidth of about 23 MHz between approximately 1,895 MHz to 1,918 MHz. Each carrier signal must support four channels in a TDMA format employing TDD for two-way communication. In particular, for each frame of time (5 ms) there are four transmit time slots (one for each channel) and four receive time slots (one for each channel). Each slot is approximately 625 micro-seconds duration with a guard time of approximately 30 micro-seconds provided within each slot.
By contrast, the European DCT system, or Digital European Cordless Telecommunication (DECT) system specifies a series of carriers spaced 1,728 MHz apart within an overall bandwidth of approximately 17.28 MHz. The DECT standard provides a network architecture in addition to an air interface physical specification and protocols. Ten carrier frequencies are employed in conjunction with twenty-four time slots per carrier and ten carriers per 20 MHz of frequency spectrum. A TDD approach is used for transmission between the cordless telephone and the base station. A transmission channel is formed via a combination of a time slots and frequencies. Transmissions occur during a ten millisecond time frame wherein each frame comprises twenty-four time slots. Twelve of the time slots are used for transmissions from the base station to the handset (downlink transmissions) while twelve are used for transmissions from the handset to the base station (uplink transmissions).
Because the DECT channels allocate an equal amount of time (and thus bandwidth) for both the uplink and downlink transmissions, the DECT TDD duplexing scheme is said to be "symmetrical" in nature. A symmetrical duplexing system is sufficient for systems (such as the DECT system) where, on the average, an equal amount of bandwidth is required for reception and transmission of information. However, symmetrical duplexing systems are inefficient in communication systems offering services requiring an asymmetric information exchange between the base station and subscriber stations. This is especially true in wireless communication systems offering wideband or "broadband" services such as voice, data and video services.
In wireless communication networks offering broadband services there is no guarantee that the uplink and downlink transmissions will have equal or symmetrical bandwidth requirements. In fact, in many scenarios currently being contemplated, it is likely that the bandwidth requirements will be unequal and asymmetrical. There are several factors driving this observation. First, the ratio of uplink and downlink bandwidth requirements is somewhat dependent upon the service provided over the link. For example, a typical telephony voice service ("POTS"-type service) has a largely symmetric uplink/downlink bandwidth requirement. However, in contrast, a broadcast video service requires a largely asymmetric uplink/downlink bandwidth requirement. Most of the information provided during a broadcast video service is uni-directional (most of the information is transmitted from the base station to the subscriber unit via the downlink, with very little or no information transmitted via the uplink). Therefore, the uplink bandwidth requirement of such a service is negligible as compared to the downlink bandwidth requirement.
Second, the ratio of uplink/downlink bandwidth desired will vary between channels in a broadband services system because each channel shall carry a multitude of diverse services Each service shall have its own unique bandwidth requirements for transmission and reception. Third, the need for symmetrical or asymmetrical communication in a channel varies depending upon the type of user. For example, in cases where a small business accesses the broadband services network for video conferencing or computer networking applications, the uplink/downlink bandwidth requirements shall be largely equal and symmetric. In contrast, in cases where a residential user accesses the broadband services network for video-on-demand (VOD) applications, the uplink/downlink bandwidth requirements shall be unequal and asymmetric. In these cases, the downlink shall require much more bandwidth than the uplink.
Therefore, a need exists for a method and apparatus which can flexibly and dynamically allocate uplink and downlink bandwidths in a time division duplexing scheme. The method and apparatus should be responsive to the needs of a particular link. The bandwidth needs may vary due to several factors, including the type of service provided over the link and the user type. The prior art systems have attempted to accommodate the need for asymmetric links by utilizing different modulation schemes for the uplink and the downlink. Under this approach, all typical bandwidth need scenarios are "averaged." This results in using a more spectrum efficient modulation scheme for the downlink. For example, a QAM-16 modulation scheme may be selected over a GMSK scheme. However, the prior art systems using this approach disadvantageously share the communication channel equally in time between the uplink and downlink transmissions. Consequently, the prior art solutions have been sub-optimal because they solve the asymmetry problem by satisfying the "average" bandwidth requirement.
However, as described above, the uplink and downlink bandwidths required in broadband networks and by broadband services are very unpredictable. In one sense, there is no average or typical scenario. Therefore, a need exists for an adaptive time division duplexing method and apparatus which can flexibly, efficiently, and dynamically allocate the uplink and downlink bandwidths for use in a broadband service network. The present invention provides such a adaptive time division duplexing method and apparatus.